Homopolyester fibers can be readily dyed with various types of dyes, and various dyes and dye compositions have been devised which are particularly suited for dyeing polyester fibers. Exemplary patents directed to dye mixtures and compositions for dyeing polyester fibers are U.S. Pat. Nos. 3,966,399, 3,989,449, 4,362,530, 4,516,979, 5,092,905, and 5,420,254. Disperse dyes may be conventionally applied to polyester fibers utilizing either a bath process or a printing process, as disclosed in U.S. Pat. No. 4,464,180. Particular problems are sometimes associated with dyeing polyesters with a selected yellow color shade, and various technology is particularly directed to this problem, as disclosed in U.S. Pat. No. 4,439,207. U.S. Pat. No. 4,840,643 discloses a trichromatic dye combination suitable for continuous dyeing of polyester fibers for allegedly resulting in excellent fastness properties.
Various processes have been utilized for dyeing both polyester fibers and polyamide fibers which are commonly used in the carpeting industry. U.S. Pat. Nos. 3,953,168, 4,125,371, 4,199,813, 4,218,217, 4,255,154, 4,579,561, 4,801,303, 5,196,031, and 5,294,231 each teach variations of techniques and processes for dyeing polyester or polyamide fibers. U.S. Pat. No. 4,185,959 discloses a technique for dyeing polyester fibers in a dye bath containing at least nine disperse dyes having different chemical structures. U.S. Pat. Nos. 4,645,510 and 5,484,456 are directed to dyeing methods particularly designed for dyeing cellulose fibers.
Various techniques and dyes have been used for improving the light fastness of dyed polyester fibers. U.S. Pat. No. 4,557,730 discloses dyes and a dyeing method particularly suitable for dyeing polyester materials used in the automotive industry. U.S. Pat. No. 4,351,641 is directed to a technique for the continuous dyeing of pile fabrics, such as carpets and upholstery fabrics, which exhibit uniform shade in the cross machine direction. Various types of polyester and cellulose dyed fibers may be subjected to a reduction clearing technique after dyeing in order to remove disperse dye molecules which have not fully diffused in the fiber. U.S. Pat. No. 4,286,961 exemplifies technology directed to reduction clearing of disperse dyes.
When dyeing polyester fibers with disperse dyes, several dyes rather than a single dye have been used to obtain very dark shades. Since each dye has its own saturation rate at which the rate of dye uptake is slower and lower, the increase in depth slows down in relation to the amount of dye available in the dye bath. A depth of shade that is much darker can be obtained with several dyes in a shorter time than when only one dye is used. Three or four dyes may thus be combined into a blended dye product and applied to the polyester to result in a darker shade at lower concentration levels and at a faster dyeing rate than one would have used if only one dye were employed in the dyeing process. If the desired yields cannot be obtained when dyeing polyester with disperse dyes at atmospheric temperatures, polyester can be readily dyed under high pressure and temperature. More than 90% of polyester fibers and its blends with cellulose fibers are dyed under pressure (e.g., 250.degree. F. to 270.degree. F.).
Deep shades of polyester are conventionally reduction cleared when needed. This process involves the treatment of the dyed fiber in an alkaline reduction bath, during which the dyes are chemically destroyed. Since the bulk of the color is trapped deep within the fiber, excellent crock fastness can still be achieved after reduction clearing the dyed polyester fibers.
Use of polypropylene in the manufacture of carpets has undergone a dramatic growth since 1980s. Polyolefins, and particularly polypropylene, are hydrophobic and difficult to dye with disperse dyes since they lack dye sites to which dye molecules may become attached. Van der Waals forces and hydrogen bonds are often given as the reason for the dyes to have affinity for the polymers. Polyolefins are also not dyeable by acid dyes since they lack basic sites with which the dye may form a salt bond. Those skilled in the art of manufacturing polypropylene fibers have long recognized that the inability to successfully dye polypropylene has limited its growth, particularly to uses where color design and flexibility are required. Accordingly, much of the polypropylene currently used in the carpet industry is colored with pigments during the extrusion process.
In the early 1990s, Lyondell Petrochemical Company developed an enhanced polypropylene resin which was successfully used to manufacture fibers suitable for the carpet industry. The new polypropylene-fibers could be disperse dyed under atmospheric pressure and conventional dyeing temperature conditions. This dyeable polypropylene composition is disclosed in U.S. Pat. Nos. 5,468,259, 5,550,192, and 5,576,366, and has been introduced by Lyondell under the KROMALON.TM. name. While polypropylene fibers developed with this technology exhibit tremendous promise, fibers and materials manufactured with this new polypropylene composition have experienced problems with respect to dyeability and fastness.
Numerous disperse dyes which are commonly used to successfully dye polyester, polyamide and acetate materials have not performed well when dyeing KROMALON. Some of these dyes do not build well into darker shades and other have insufficient fastness properties. Polyester and polyamide fibers may be easily dyed with disperse dyes since there is mechanical bonding of the dye to the expanding fiber. In polypropylene, this mechanical bonding does not occur since polypropylene does not absorb well and swell in water. The dye saturation level when dyeing polypropylene is thus very low compared to the dye saturation level when dyeing polyester, and dyed polypropylene generally exhibits poor crock fastness and rubbing fastness compared to dyed polyester. In general, the saturation curve for KROMALON is thus markedly different than the saturation curve for polyesters and polyamides.
Since the saturation concentration from many dyes is quite low when dyeing KROMALON compared to the saturation concentration for the same dyes on polyester, attempts to dye KROMALON in a very dark shade frequently result in 50% or more of the dye not exhausting on the KROMALON material, thereby resulting in dyed fibers which have very poor crock and rubbing fastness. While polyesters may be conventionally dyed to very dark depths with disperse dyes, extreme difficulties are thus encountered when trying to reach the same dark depths with adequate fastness when dyeing KROMALON.
As noted above, polyester fibers which are disperse dyed may be easily reduction cleared to remove the loose dye stuff. Reduction clearing of polyester typically occurs at 160.degree. F. to 180.degree. F. utilizing mixture of caustic soda and sodium hydrosulfite. This same reduction clearing if performed on polypropylene would adversely alter the shade to a significant degree. The reductive force of the treatment of polyester happens only on the outer skin of the fiber. Polypropylene is not as dense as polyester, and is thus more porous and accessible so that the reduction treatment penetrates inside the fiber, thereby destroying the color.
KROMALON accordingly has a dye fixation mechanism which is markedly dissimilar to the dye fixation mechanism used to dye both polyesters and polyamides. High temperature dyeing of KROMALON does not seem to result in the significant improvement in color yield which is experienced when dyeing polyester at high temperatures. Dyeing of polyesters is frequently enhanced with the use of carriers. Typical carriers are organic compounds, such as butyl benzoate, bi-phenyl, trichlorobenzene, that can be emulsified in water and thus permit faster dyeing. Carriers penetrate the fiber, often swelling the fiber and aiding the passage of the disperse dye across the dye dispersion fiber interface and into the fiber. Carriers commonly used with disperse dyes for dyeing polyester are not effective to any significant degree on the KROMALON fibers, and those that help have other limitations such as a drastic drop in light fastness or, because of their strong and offensive odors, cannot be used for many applications.
The disadvantages of the prior art are overcome by the present invention. An improved process is hereinafter disclosed for selecting dyes, and for combining disperse dyes which are able to reliably dye polyolefins, and particularly polypropylene. The improved method of dyeing KROMALON according to this invention allows the dye operator to obtain the desired shade and darkness of the dyed substrate without the dyed fibers thereafter exhibiting crocking problems.